1. Field of the Invention:
The present invention relates to a device for ensuring simply and accurately the tensioning of a cylindrical body, such as a rod or a casing, which can elongate elastically when it is made to undergo a tensile force. Although the use described below relates essentially to the tensioning of a casing for an oil well, the invention can be used for tensioning and maintaining this tension in any body capable of undergoing elastic elongation.
2. Description of the Prior Art:
The suspension of the casings on the wellhead in an oil-well installation is usually obtained by means of a wedge mechanism which, bearing on an inner conical surface of the wellhead, highly compresses the outside of the casing in order to obtain the suspension of the latter. In most cases, in order to install this wedge mechanism, it is necessary to remove the blow-out preventer block and use a wellhead with stacked receptacles, each of them being associated with each casing, in such a way as to make it easier to cut the casing above the wedge mechanism. After the casing has been cut and chamfered, a sealing element is slipped between the bore in the receptacle of the wellhead and the casing in order to isolate the annular space present between the casing and the wellhead.
The development of underwater wellheads has given rise to integral wellheads possessing a suspension collar for the casing. In this case, the casing is connected by screwing to a collar which has a substantially radial bearing shoulder, thus avoiding the need to use a wedge mechanism and eliminating the annular compression stresses generated in the casing by this wedge mechanism. An annular sealing element can then be installed remotely between the suspension collar. This arrangement makes it possible to avoid disconnecting the blow-out preventer, and the latter can then ensure the protection of the operations of suspension, cementing and sealing the casing. Because the casing does not have to be cut, receptacles for each casing dimension can be connected in one piece to form an integral wellhead.
This arrangement with a suspension collar and an integral wellhead is not suitable when tension must be introduced into the casing after cementation. It is then necessary to return to the mechanism with locking wedges.